Heparin causes thrombocytopenia in 1 - 3 per cent of patients and leads to thrombosis in about half of these. There have been several laboratory tests to diagnose this problem, but none of them is perfect and some are very labor intensive. Recently a new assay method has been published that offers the promise of high sensitivity and specificity but with relatively little labor. In order to collect a sufficient number of patient samples to study this new assay, we have collaborated with physicians at Washington Hospital Center, where heparin-induced thrombocytopenia (HIT) is much more common than here. Thirty-four patients (from a goal of 40) have been accrued with a spectrum of clinical probability of actually having HIT. Samples from these patients have been tested with the new laboratory test for HIT as well as the time-honored but difficult assay that we currently consider the best. Unfortunately there have been several discrepancies between the tests. The clinical probability scores of the patients have been blinded until all of the testing is complete.